The present invention relates to electrical machines. More specifically, the present invention relates to a permanent magnet generator.
Many aircraft include high-speed electrical generators for generating power during flight. Ac power is generated and rectified to dc power, and the dc power is supplied through a dc distribution system to on-board electronics such as radar, vapor cycle compressors, flight control electronics and electromechanical/electrohydrostatic actuators. The electrical generators may be wound field synchronous machines, switched reluctance machines, permanent magnet machines, or other types of machines. For high-speed applications, a so-called xe2x80x9cTwo Pole Toothlessxe2x80x9d permanent magnet machine is desirable because of its robust rotor design and a low magnetic spring rate associated with a large air gap.
Despite their advantages, permanent magnet machines are not preferred for aircraft generator application because they could not survive short circuits applied to their terminals. Because of their low impedance, short circuit currents in excess of the machine current rating can flow under short circuit applications, causing excessive heat build up in the machine stator windings. A short in the stator windings can literally melt the windings and destroy a machine. If a machine cannot survive the short circuit conditions, it cannot recover upon short circuit removal, and it cannot deliver power to the aircraft.
There is a need for a high-speed permanent magnet generator that can gracefully survive short circuit conditions.
According to one aspect of the present invention, a system includes a permanent magnet machine; a dc link; and a control including an inverter coupled between the dc link and the terminals of the machine. The control regulates dc link voltage by adjusting the load angle and the magnitude of the inverter ac terminal voltage with respect to the emf of the machine.
A machine having a high reactance will not become damaged if its windings are shorted during full-speed operation, since the machine is designed such that the thermal ratings under both short circuit and rated power conditions are essentially identical. Because such a permanent magnet machine can xe2x80x9cgracefullyxe2x80x9d survive shorts, it can be used, in conjunction with the control system and its associated inverter, to generate dc electrical power for use on aircraft.